Recently, as slimness of TV products including LED TV, or the like, in addition to a portable terminal such as a mobile phone, a smart phone, a notebook, or the like, is accelerated, a piezoelectric speaker has been in the limelight as alternatives for an existing dynamic speaker using a magnet coil. The piezoelectric speaker may be manufactured thinner and lighter and may consume less power, as compared with the existing dynamic speaker, such that it has emerged as a speaker technology for the future. In particular, as the portable terminal requires to be small, slim, and light, the applications of the piezoelectric speaker have been actively searched.
However, despite the above-mentioned merits, the piezoelectric speaker has difficulty in commercialization because the piezoelectric speaker outputs low sound pressure and is difficult to reproduce low frequency as compared with the dynamic speaker of the related art. An example of the piezoelectric speaker of the related art may include a piezoelectric speaker manufactured by using a piezoelectric oscillator or attaching a piezoelectric disk to a top of a metal diaphragm, a film type piezoelectric speaker such as polyvinylidene fluoride (PVDF), a micro piezoelectric speaker manufactured by using a silicon mechanical electronic micromachined system (MEMS) process, or the like.
As the piezoelectric speaker of the related art, the piezoelectric speaker using the piezoelectric oscillator is manufactured by attaching the piezoelectric oscillator to the outside of an oscillator panel and uses a principle of generating sound by oscillating the oscillation panel by the piezoelectric oscillator. The piezoelectric speaker using the piezoelectric oscillator needs to transfer the oscillation of the piezoelectric material to the oscillator panel via the elastomer for transferring oscillation, thereby requiring very large oscillation of the piezoelectric material and the relatively larger oscillator panel than the piezoelectric oscillator. In addition, in the case of the piezoelectric speaker using the piezoelectric oscillator, unnecessary resonance may occur during the process of transferring oscillation, such that a peak-dip of the output sound pressure may occur and distortion of sound may occur to degrade the sound quality.
As another piezoelectric speaker of the related art, the piezoelectric speaker manufactured by attaching the piezoelectric disk to the top of the metal diaphragm has a structure of bonding the piezoelectric material to the top of an oscillation thin film made of a metal plate or an alloy, or the like, by using a bonding material and uses a principle of reproducing sound by displacing the metal diaphragm through input signals applied to the piezoelectric material. The piezoelectric speaker can be miniaturized and can be driven at low voltage due to the excellent oscillation transferring performance of the metal diaphragm, as compared with the piezoelectric speaker using the piezoelectric oscillator of the related art. However, the piezoelectric speaker using the metal diaphragm has the relatively thicker oscillation thin film than the piezoelectric material, such that the piezoelectric speaker using the metal diaphragm may output low sound pressure and may be difficult to reproduce low frequency. Further, the piezoelectric speaker using the metal diaphragm may have difficulty in reproducing the low frequency of 1 kHz or less due to a very high elastic modulus of metal. In addition, in the case of the oscillation thin film using the existing metal plate, a rich sound field effect may not be implemented well due to a cold and sharp tone of a metal material and unnecessary resonance may occur due to the frame supporting the oscillation thin film to distort sound.
As another piezoelectric speaker of the related art, the film type piezoelectric speaker using the piezoelectric film material uses a principle of forming electrodes on the top and bottom of the metal diaphragm using the piezoelectric film material such as PVDF and applying voltage to the electrodes to generate sound. The film type piezoelectric speaker is manufactured in a structure in which a polymer conductive layer is formed on both sides of the piezoelectric film and electrodes are formed in a form extending along an edge thereof and then, terminals are formed so as to apply voltage to the electrodes. The film type piezoelectric speaker has a low piezoelectric constant of a piezoelectric material to generate a small displacement, such that the film type piezoelectric speaker needs to be manufactured as a large-area piezoelectric speaker and requires a relatively larger oscillation thin film than other speakers of the related art.
The piezoelectric speaker of the related art outputs the lower sound pressure than the dynamic speaker, and in particular, may be very difficult to reproduce the low frequency. In addition, the piezoelectric speaker of the related art has a narrow frequency reproducing band to degrade the sound quality and requires a sufficiently thin or large diaphragm so as to perform the low frequency reproduction, such that the piezoelectric speaker may not be easily miniaturized when considering the high output sound pressure and the low frequency reproduction.